The embodiments herein relate generally to selective mining, and more particularly, to a process for the enhanced recovery of solution mined minerals.
Solution mining is a mining method in which the mining of desired materials is achieved by the injection of a water, or a lean water solution, underground and into a known ore bed containing the desired mineral in a grade concentration that has been determined to be economically feasible for mining. The mineral is dissolved into the water and the rich water solution flows by pump pressure back to the surface and into a mineral recovery processing plant. A solution mining project can be, and has been, an alternative to conventional underground mining projects in which miners and mining equipment work underground to extract and bring to the surface ore in a solid form.
Conventional solution mining processes typically have lower equipment, personal, and maintenance costs than conventional underground mining. However, the energy requirement of the solution methods can be higher due to the need to pump and heat the injection water. Both the heating and pumping energy required is influenced greatly by the water circulation rate required for mining and process recovery. This rate is, in turn, influenced strongly by the mineral concentration difference between the lean injection water solution and the rich mine production water solution. A leaner injection solution and also a richer production solution would result in a reduced circulation rate required to meet the desired product production rate. This relationship mathematically increases exponentially with increased concentration difference.
Sylvinite ore is the most common type of potash containing ore and principally consists of potash minerals sylvinite (KCl) and the mineral halite (NaCl), but can also contain minor amounts of carnallite (KCl.MgCl.6H2O), as explained in Garrett, D. E., “Potash: Deposits, Processing, Properties, and Use,” 1st Ed., Chapman & Hall, Chapter 1, page 1 (1996). The potash grade for commercially viable sylvinite mining for potash is typically about 20 wt. % potash with the remainder being principally halite.
Conventional Solution mining of sylvinite for potash is performed as primary mining, wherein the well injector fluid is almost pure water. This allows for a large KCl concentration difference and a reduced circulation rate, but due to a greater solubility for NaCl than KCl, more NaCl is mined than KCl, as shown graphically in FIG. 6. Typically, about 1.5 tons of NaCl are mined for every ton of KCl, which requires that the unwanted NaCl salt be removed from the solution and stockpiled so that the KCl content of the potash product can be recovered and processed with the typical 95 wt. % or greater product purity. Removal of the NaCl requires evaporation of the water, which is then condensed and used as the principal source of water in the well injection system. The evaporation is typically carried out in large industrial evaporators made of expensive corrosion resistant alloy metals, and additional separation equipment, such as hydroclones, centrifuges, and clarifiers, may also be needed to remove the precipitated NaCl salt formed in the evaporators. Thus, primary mining may require added equipment and energy.
Because of the added equipment and energy expenses related to primary mining, solution mining of sylvinite ore is also performed with an injection brine saturated with NaCl salt and under saturated with KCl, resulting in the unwanted NaCl being left in the mine and allowing for the mining of principally just the KCl contained in the sylvinite. This is known in the industry as secondary mining and takes place after sufficient surface area has been created in primary mined caverns. However, there is still the need to keep primary mining going to continually create and make available mine caverns for secondary mining. A typical conventional solution mining facility with both primary and secondary mining operations will produce about ⅔ of the potash from primary mining and about ⅓ from secondary mining.
With the advent of new well drilling techniques, such as horizontal drilling, to create larger amounts of surface area in the initial wellfield caverns, solution mining operations can now be constructed in which the entire plant production is produced with a saturated NaCl injection solution. This is known in the industry as selective mining.
The solution mining process currently in use for mineral recovery (as described in U.S. Patent Application Publication No. 2015/0044113 to Batty et al.) involves the evaporation of water in a multiple effector mechanical recompression evaporator prior to the cooling crystallization step. The evaporator precipitates the NaCl in a solid form and thereby separates it from the KCl that remains in solution. Water vapor from the evaporator is condensed and returned to the mine for continued mining. As described above, this is typically referred to as primary mining in the solution mining industry, which results in the mining of the unwanted NaCl salt, requiring larger capital and energy costs.
Crystallization of the KCl occurs next in a cooling crystallization step. Chilled water can be used for cooler crystallization temperatures during these processes, resulting in increased KCl recovery. However, conventionally, the chilled water is produced using compression chilling with refrigeration compressors. The electrical energy requirement for the compressors can increase the electrical requirements for the entire solution mining and process plant by as much as about 30%. This additional energy requirement in itself adds additional operating costs, but the cost can also be increased if the electrical energy is produced by a hydrocarbon burning utility company, wherein overall generation efficiency is limited to about 50% due to the need for cooling water condensing of the turbine exhaust steam.
In some parts of the world where cold climates persist longer during the year, the use of large outdoor cooling ponds provide the cooling for the KCl potash separation from the NaCl salt. See Garrett, page 316. In such cases, the precipitation of the solid KCl crystals generally takes place during the winter months, and dredges are used to slurry the precipitated KCl crystals to the process plant for potash recovery. Although the energy requirement for pond cooling is provided by nature, the yearly variance in the amount of KCl crystals generated requires the mining and recovery equipment be oversized for the maximum amount of crystal production that occurs during the coldest months. This can add as much as 25-50% to the size of the wellfield pumping and piping equipment, in addition to the plant recovery equipment that is needed for a cooling crystallizer system. Cooling ponds also require environmental monitoring and operating costs for pond leakage identification and control.
Therefore, what is needed is a process for the enhanced recovery of solution mined minerals that simultaneously reduces the cost of production for the recovery of the solution mined minerals by, for example, reducing the amount of energy and water required.